Lead-free solder alloy and a manufacturing process of electric and electronic apparatuses using such a lead-free solder alloy

ABSTRACT

A lead-free solder alloy composition containing Sn, Ag and Bi, with respective concentrations set such that the lead-free solder alloy has a melting temperature lower than a predetermined heat-resistant temperature of a work to be soldered.

This is a Division of application Ser. No. 08/526,929 filed Sep. 12,1995, now U.S. Pat. No. 6,184,475. The disclosure of the priorapplication is hereby incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

The present invention generally relates to manufacturing of electric andelectronic apparatuses and more particularly to a solder alloy ofvarious forms used for soldering electric and electronic components, aswell as to a soldering process and further to a rig used for such asoldering process. In particular, the present invention relates to alead-free solder alloy that contains no substantial amount of lead (Pb).

Solder alloys are characterized by low melting temperatures and provideexcellent electric as well as mechanical properties. Thus, solder alloysof various forms, including solder powders and solder pastes, are usedfor mounting electronic components on a printed circuit board.

Meanwhile, conventional solder alloys contain Pb. As Pb is toxic againstbiological bodies, it has been necessary to take precautionary measurewhen conducting such a soldering process, while such a precautionarymeasure increases the cost of the products produced as a result of thesoldering. Thus, there is a demand for a lead-free solder alloy that issuitable for use in various soldering processes including automatedsoldering process.

In the automated soldering process of electronic components, severaltypes of solder alloys are used conventionally. A representative exampleis a solder alloy known as Sn63—Pb37, wherein the solder alloy contains63 wt % of Sn and 37 wt % of Pb. This material causes an eutecticmelting at a melting temperature of 183° C. Another typical example is asolder alloy known as Sn62—Pb36—Ag2, wherein the solder alloy contains62 wt % of Sn, 36 wt % of Pb and 2 wt % of Ag. The solder alloy forms aneutectic system characterized by a melting temperature of 179° C. Asthese solder alloys have low melting temperatures and provide excellentmechanical properties in terms of tensile strength and elongation aswell as excellent electrical properties such as low resistance, they areused extensively for various automated soldering processes.

Meanwhile, there is a tendency of increasing public regulations againstthe use of Pb in view of human health and in view of environmentalprotection. Under such circumstances, various efforts have been made fordeveloping a substitute solder alloy that is free from Pb.

As the material for use in assembling electric and electronicapparatuses, such a substitute solder alloy is required to have a lowmelting temperature such that the soldered electric or electroniccomponent experiences little degradation of performance caused by theheat at the time of soldering. Further, such a substitute solder alloyshould have an excellent mechanical strength comparable to that of aconventional solder alloy that contains Pb.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to providea novel and useful solder alloy of various forms as well as a solderingprocess wherein the foregoing problems are eliminated.

Another and more specific object of the present invention is to providea solder alloy free from Pb and still having a sufficiently low meltingtemperature, high conductivity and high mechanical strength.

Another object of the present invention is to provide a lead-free solderalloy composition comprising Sn, Bi and In, said solder alloy containingSn, Bi and In with respective concentrations set such that saidlead-free solder alloy composition has a melting temperature lower thana predetermined heat-resisitant temperature of a work to be soldered.

Another object of the present invention is to provide a method forsoldering a work, comprising the steps of:

reflowing a lead-free solder alloy containing therein Sn, Bi and In withrespective contents set such that said solder alloy has a meltingtemperature lower than a predetermined heat-resistant temperature ofsaid work, said step of reflowing including a step of heating saidsolder alloy to a temperature higher than said melting temperature; and

cooling said work at a part where a soldering has been made to solidifysaid lead-free solder alloy.

Another object of the present invention is to provide a lead-free solderalloy composition containing: Bi with a concentration not exceeding 60.0wt %; In with a concentration not exceeding 50.0 wt %; one or moreelements selected from a group consisting of Ag, Zn, Ge, Ga, Sb and P,with a concentration equal to or larger than 1.0 wt % but lower than 5.0wt %; and Sn as a balancing component of said lead-free solder alloy.

Another object of the present invention is to provide a solderingprocess of a work, comprising the steps of:

reflowing a lead-free solder alloy containing therein: Bi with aconcentration not exceeding 60.0 wt %; In with a concentration notexceeding 50.0 wt %; one or more elements selected from a groupconsisting of Ag, Zn, Ge, Ga, Sb and P, with a concentration equal to orlarger than 1.0 wt % but lower than 5.0 wt %; and Sn as a remainingcomponent of said solder alloy; and

cooling said work at a part where a soldering is made to solidify saidlead-free solder alloy.

Another object of the present invention is to provide a lead-free solderalloy composition containing Sn, Ag and Bi, with respectiveconcentrations set such that said lead-free solder alloy has a meltingtemperature lower than a predetermined heat-resistant temperature of awork to be soldered.

Another object of the present invention is to provide a method ofsoldering a work, comprising the step of:

reflowing a lead-free solder alloy containing therein Sn, Ag and Bi withrespective contents set such that said lead-free solder alloy has amelting temperature lower than a predetermined heat-resisitanttemperature of said work, said step of reflowing including a step ofheating said lead-free solder alloy to a temperature higher than saidmelting temperature; and

cooling said work at a part where a soldering is made to solidify saidlead-free solder alloy.

Another object of the present invention is to provide a lead-free solderpowder comprising:

a plurality of lead-free solder particles each having a generallyspherical shape with a diameter of 20-60 μm;

each of said lead-free solder particles containing Sn, Bi and In, withrespective concentrations set such that said lead-free solder particlehas a melting temperature lower than a predetermined heat-resistanttemperature of a work to be soldered.

Another object of the present invention is to provide a lead-free solderpowder comprising:

a plurality of lead-free solder particles each having a generallyspherical shape with a diameter of 20-60 μm;

each of said lead-free solder particles containing Bi with aconcentration not exceeding 60.0 wt %; In with a concentration notexceeding 50.0 wt %; one or more elements selected from a groupconsisting of Ag, Zn, Ge, Ga, Sb and P, with a concentration equal to orlarger than 1.0 wt % but lower than 5.0 wt %; and Sn as a remainingcomponent of said lead-free solder particle.

Another object of the present invention is to provide a lead-free solderpowder comprising:

a plurality of lead-free solder particles each having a generallyspherical shape with a diameter of 20-60 μm;

each of said lead-free solder particles containing Sn, Ag and Bi, withrespective concentrations set such that said lead-free solder alloy hasa melting temperature lower than a predetermined heat-resistanttemperature of a work to be soldered.

Another object of the present invention is to provide lead-free solderpaste, comprising:

a lead-free solder powder comprising a plurality of lead-free solderparticles each having a generally spherical shape with a diameter of20-60 μm; each of said lead-free solder particles containing Sn, Bi andIn, with respective concentrations set such that said lead-free solderparticle has a melting temperature lower than a predeterminedheat-resistant temperature of a work to be soldered, said solder powderbeing contained with a proportion of 80.0-95.0 wt %; and

a mixture of an amine halide, a polyhydric alcohol and a polymer, with aproportion of 20.0-5.0 wt %.

Another object of the present invention is to provide a lead-free solderpaste, comprising:

a lead-free solder powder comprising a plurality of lead-free solderparticles each having a generally spherical shape with a diameter of20-60 μm; each of said lead-free solder particles containing Bi with aconcentration not exceeding 60.0 wt %; In with a concentration notexceeding 50.0 wt %; one or more elements selected from a groupconsisting of Ag, Zn, Ge, Ga, Sb and P, with a concentration equal to orlarger than 1.0 wt % but lower than 5.0 wt %; and Sn as a remainingcomponent of said solder alloy; said lead-free solder powder beingcontained with a proportion of 80.0-95.0 wt %; and

a mixture of an amine halide, a polyhydric alcohol and a polymer, with aproportion of 20.0-5.0 wt %.

Another object of the present invention is to provide a lead-free solderpaste, comprising:

a lead free solder powder comprising a plurality of lead-free solderparticles each having a generally spherical shape with a diameter of20-60 μm; each of said lead-free solder particles containing Sn, Ag andBi, with respective concentrations set such that said lead-free solderparticle has a melting temperature lower than a predeterminedheat-resistant temperature of a work to be soldered; and

a mixture of an amine halide, a polyhydric alcohol and a polymer, with aproportion of 20.0-5.0 wt %.

Another object of the present invention is to provide a lead-free solderpaste, comprising:

a lead-free solder powder comprising a plurality of lead-free solderparticles each having a generally spherical shape with a diameter of20-60 μm; each of said lead-free solder particles containing Sn, Bi andIn, with respective concentrations set such that said lead-free solderpowder has a melting temperature lower than a predeterminedheat-resistant temperature of a work to be soldered, said lead-freesolder powder being contained with a proportion of 80.0-95.0 wt %; and

a mixture of an organic acid, a polyhydric alcohol and a polymer, with aproportion of 20.0-5.0 wt %.

Another object of the present invention is to provide a lead-free solderpaste, comprising:

a lead-free solder powder comprising a plurality of lead-free solderparticles each having a generally spherical shape with a diameter of20-60 μm; each of said lead-free solder particles containing Bi with aconcentration not exceeding 60.0 wt %; In with a concentration notexceeding 50.0 wt %; one or more elements selected from a groupconsisting of Ag, Zn, Ge, Ga, Sb and P, with a concentration equal to orlarger than 1.0 wt % but lower than 5.0 wt %; and Sn as a remainingcomponent of said solder alloy; said lead-free solder powder beingcontained with a proportion of 80.0-95.0 wt %; and

a mixture of an organic acid, a polyhydric alcohol and a polymer, with aproportion of 20.0-5.0 wt %.

Another object of the present invention is to provide a lead-free solderpaste, comprising:

a lead-free solder powder comprising a plurality of lead-free solderparticles each having a generally spherical shape with a diameter of20-60 μm; each of said lead-free solder particles containing Sn, Ag andBi, with respective concentrations set such that said lead-free solderalloy has a melting temperature lower than a predeterminedheat-resistant temperature of a work to be soldered; and

a mixture of an organic acid, a polyhydric alcohol and a-polymer, with aproportion of 20.0-5.0 wt %.

Another object of the present invention is to provide a printed circuitboard, comprising:

a substrate;

a conductor pattern provided on said substrate; and

a lead-free solder alloy covering said conductor pattern, said lead-freesolder alloy containing Sn, Bi and In, with respective concentrationsset such that lead-free said solder alloy has a melting temperaturelower than a predetermined heat-resistant temperature of a component tobe soldered upon said substrate.

Another object of the present invention is to provide printed circuitboard, comprising:

a substrate;

a conductor pattern provided on said substrate; and

a lead-free solder alloy covering said conductor pattern, said lead-freesolder alloy containing: Bi with a concentration not exceeding 60.0 wt%; In with a concentration not exceeding 50.0 wt %; one or more elementsselected from a group consisting of Ag, Zn, Ge, Ga, Sb and P, with aconcentration equal to or larger than 1.0 wt % but lower than 5.0 wt %;and Sn as a remaining component of said lead-free solder alloy.

Another object of the present invention is to provide a printed circuitboard, comprising:

a substrate;

a conductor pattern provided on said substrate; and

a lead-free solder alloy covering said conductor pattern, said lead-freesolder alloy containing: Sn, Ag and Bi, with respective concentrationsset such that said lead-free solder alloy has a melting temperaturelower than a predetermined heat-resistant temperature of a component tobe soldered upon said substrate.

Another object of the present invention is to provide an electroniccomponent, comprising:

an electronic component body;

an electrode projecting from said electronic component body; and

a lead-free solder alloy covering said electrode, said lead-free solderalloy containing Sn, Bi and In, with respective concentrations set suchthat said lead-free solder alloy has a melting temperature lower than apredetermined heat-resistant temperature of said electronic component.

Another object of the present invention is to provide an electroniccomponent, comprising:

an electronic component body;

an electrode projecting from said electronic component body; and

a lead-free solder alloy covering said electrode, said lead-free solderalloy containing: Bi with a concentration not exceeding 60.0 wt %; Inwith a concentration not exceeding 50.0 wt %; one or more elementsselected from a group consisting of Ag, Zn, Ge, Ga, Sb and P, with aconcentration equal to or larger than 1.0 wt % but lower than 5.0 wt %;and Sn as a remaining component of said lead-free solder alloy.

Another object of the present invention is to provide an electroniccomponent, comprising:

an electronic component body;

an electrode projecting from said electronic component body; and

a lead-free solder alloy covering said conductor pattern, said lead-freesolder alloy containing: Sn, Ag and Bi, with respective concentrationsset such that said lead-free solder alloy has a melting temperaturelower than a predetermined-heat-resistant temperature of a component tobe soldered upon said substrate.

Another object of the present invention is to provide an electronicapparatus, comprising:

a substrate;

a conductor pattern provided on said substrate;

an electronic component mounted upon said substrate in electricalconnection with said conductor pattern, said electronic component havingan electrode projecting therefrom; and

a lead-free solder alloy connecting said electrode to said conductorpattern, said lead-free solder alloy containing Sn, Bi and In, withrespective concentrations set such that said lead-free solder alloy hasa melting temperature lower than a predetermined heat-resistanttemperature of said electronic component.

Another object of the present invention is to provide an electronicapparatus, comprising:

a substrate;

a conductor pattern provided on said substrate;

an electronic component mounted upon said substrate in electricalconnection with said conductor pattern, said electronic component havingan electrode projecting therefrom; and

a lead-free solder alloy connecting said electrode to said conductorpattern, said lead-free solder alloy containing: Bi with a concentrationnot exceeding 60.0 wt %; In with a concentration not exceeding 50.0 wt%; one or more elements selected from a group consisting of Ag, Zn, Ge,Ga, Sb and P, with a concentration equal to or larger than 1.0 wt % butlower than 5.0 wt %; and Sn as a remaining component of said lead-freesolder alloy.

Another object of the present invention is to provide an electronicapparatus, comprising:

a substrate;

a conductor pattern provided on said substrate;

an electronic component mounted upon said substrate in electricalconnection with said conductor pattern, said electronic component havingan electrode projecting therefrom; and

a lead-free solder alloy connecting said electrode to said conductorpattern, said lead-free solder alloy containing Sn, Ag and Bi, withrespective concentrations set such that said lead-free solder alloy hasa melting temperature lower than a predetermined heat-resistanttemperature of said electronic component.

Another object of the present invention is to provide a soldering rigfor soldering a work, comprising:

soldering unit for soldering a work by causing a reflow of a lead-freesolder; and

a cooling unit for cooling said work at a part where a soldering hasbeen made, to solidify said lead-free solder.

According to the present invention as set forth above, one can obtain asolder alloy free from Pb while maintaining excellent mechanicalstrength in the solidified solder alloy. Thereby, the problem of hazardto biological bodies as well as the problem of environmental pollutionare successfully eliminated. Further, by optimizing the composition ofthe solder alloy, it is possible to reduce the melting temperature ofthe solder alloy lower than a melting temperature of a conventionalsolder alloy that contains Pb, while maintaining sufficient mechanicalstrength. Thereby, the damage applied to the work or electroniccomponent as a result of soldering is reduced. Associated with thereduced temperature of soldering, the preparation of the work forsoldering is substantially simplified, and the cost of the work is alsoreduced by using less expensive materials. By cooling the solder alloyrapidly, it is possible to maximize the elongation of the solidifiedsolder alloy.

Other objects and further features of the present invention will becomeapparent from the following detailed description when read inconjunction with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram summarizing the effect of In added to a solder alloyof a Sn—Bi eutectic system in the form of a table;

FIG. 2 is a diagram summarizing the effect of Bi added to a solder alloyof a Sn—Bi—In ternary system in the form of a table;

FIG. 3 is a diagram summarizing the effect of various elements added toa solder alloy of a Sn—Bi—In ternary system in the form of a table;

FIG. 4 is a diagram summarizing the effect of Bi added to a solder alloyof a Sn—Ag eutectic system in the form of a table;

FIGS. 5A-5I are diagrams showing a particle of a lead-free solder powderaccording to an embodiment of the present invention;

FIG. 6 is a diagram showing the composition of a lead-free solder pasteaccording to another embodiment of the present invention in the form ofa table;

FIG. 7 is a diagram showing the composition of a lead-free solder pasteaccording to other embodiment of the present invention in the form of atable;

FIG. 8 is a diagram showing the construction of a printed circuit boardaccording to still other embodiment of the present invention;

FIG. 9 is a diagram showing the construction of a printed circuit boardaccording to still other embodiment of the present invention;

FIG. 10 is a diagram showing the construction of a semiconductor devicemounted upon a printed circuit board according to still other embodimentof the present invention;

FIG. 11 is a diagram showing the mechanical property of the lead-freealloy of various embodiments of the present invention in the form of atable;

FIG. 12 is a diagram showing the detailed experimental result conductedfor a sample included in FIG. 11, in the form of a table;

FIG. 13 is a diagram showing the detailed experimental result conductedfor another sample included in FIG. 11, in the form of a table;

FIG. 14 is a diagram showing the detailed experimental result conductedfor other sample included FIG. 11, in the form of a table;

FIG. 15 is a diagram showing the detailed experimental result conductedfor other sample included in FIG. 11, in the form of a table;

FIG. 16 is a diagram showing the detailed experimental result conductedfor other sample included in FIG. 11, in the form of a table;

FIG. 17 is a diagram showing the relationship between elapsed time andthe load for the sample of FIG. 12;

FIG. 18 is a diagram showing the relationship between elapsed time andthe load for the sample of FIG. 13;

FIG. 19 is a diagram showing the relationship between elapsed time andthe load for the sample of FIG. 14;

FIG. 20 is a diagram showing the relationship between elapsed time andthe load for the sample of FIG.15;

FIG. 21 is a diagram showing the relationship between elapsed time andthe load for the sample of FIG. 16;

FIG. 22 is a diagram showing the state of fracture of the sample of FIG.12;

FIG. 23 is a diagram showing the state of fracture of the sample of FIG.13;

FIG. 24 is a diagram showing the state of fracture of the sample of FIG.14;

FIG. 25 is a diagram showing the state of fracture of the sample of FIG.15;

FIG. 26 is a diagram showing the state of fracture of the sample of FIG.16;

FIG. 27 is a diagram showing the soldering process according to stillother embodiment of the present invention in the form of a flowchart;

FIG. 28 is a diagram showing a soldering rig according to a still otherembodiment of the present invention;

FIG. 29 is a diagram showing the construction of a cooling unit of thesoldering rig of FIG. 28;

FIG. 30 is a diagram showing the construction of another cooling unit ofthe soldering rig of FIG. 28; and

FIG. 31 is a diagram showing the construction of still other coolingunit of the soldering rig of FIG. 28.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described with reference tothe preferred embodiments.

In the present invention, the inventor has conducted a series ofexperiments to prepare various solder alloys free from Pb and to examinethe properties thereof. As a result, it has been discovered that alead-free solder alloy containing Sn, Bi and In as well as a lead-freesolder alloy containing Sn, Ag and Bi, show mechanical as well aselectrical properties comparable or even superior to those of theconventional solder alloy that contains Pb.

Thus, the inventor of the present invention has conducted detailedexperiments on the lead-free solder alloy containing Sn, Bi and In aswell as on the lead-free solder alloy containing Sn, Ag and Bi in searchof the optimum composition of the solder alloy. Further, experimentshave been conducted also on the alloys in which other metal or non-metalelements are added.

First, a description will be made on the experiments about the solderalloy of the Sn—Bi—In ternary system with reference to FIGS. 1 and 2,wherein FIGS. 1 and 2 are tables that summarize the result of theexperiments on the tensile strength, percentage of elongation,time-to-failure, fracture surface morphology and the melting temperaturefor various compositions of the solder alloy. Further, FIG. 3 shows atable summarizing the result of the similar experiments conducted forsolder alloys in which other metal as well as non-metal elements areadded.

Before going to the evaluation of the experimental results, explanationwill be made on the testing process and testing apparatuses used in theexperiments.

In the experiments, standard test pieces prescribed in the JIS (JapaneseIndustrial Standard) were produced from solder alloys of variouscompositions, and the test pieces thus produced were subjected to atensile test for mechanical properties such as tensile strength,percentage of elongation, time-to-failure and fracture surfaceobservation. Further, the melting temperature of the solder alloy wasmeasured by using a thermocouple.

More specifically, the test pieces were produced according to the JIStype 7 prescription for tensile tests. The test piece thus produced hada cross sectional area of 40 mm² and a gauge length of 30 mm. The testpieces were prepared by melting an alloy of Sn, Bi and In or an alloycontaining further impurity elements in a furnace held at 400° C.,wherein the molten solder alloy thus prepared was poured into a moldcarrying therein a cavity with a shape corresponding to the JIS type 7test piece prescription.

In the tensile test, a test rig of Instron Model 4206 was used. The testpiece was set on the rig, and the test was conducted by pulling the testpiece with a fixed rate of 0.5 mm/min while recording the tensile loadand the elongation of the test piece. Upon occurrence of the failure,the tensile strength and the percentage of elongation were calculatedbased upon the record. Further, a discrimination was made whether thefracture was a ductile one or brittle one based upon the observation ofthe fracture surface morphology.

The measurement of the melting temperature or liquidus temperature ofthe solder alloy, on the other hand, was made by causing a melting ofthe solder alloy, followed by a natural cooling. During the process ofnatural cooling, the temperature profile was measured by means of athermocouple inserted into the molten solder alloy.

As already noted, FIGS. 1 through 3 summarize the result of the test interms of the tensile strength, elongation, time-to-failure, fracturesurface and the melting temperature. When the properties observed weresatisfactory for a solder alloy, an open circle mark was given as anindication of positive evaluation. When the properties wereunsatisfactory, on the other hand, a cross mark was given as anindication of negative evaluation. In the present test, the evaluationwas made based upon a standard that: (1) the solder alloy should have atensile strength of at least 2 kg/mm²; (2) the solder alloy should havean elongation of at least 30%; and (3) the solder alloy should have amelting temperature equal to or lower than 155° C.

Referring to FIG. 1, it will be noted that the content of In is changedvariously in the ternary alloy of the Sn—Bi—In system while maintainingthe content of Bi generally constant. As will be noted in FIG. 1, thesolder alloy provides a satisfactory tensile strength as long as the Incontent in the alloy is less than 50 wt %. When the In content exceeds50 wt % as in the case of the comparative examples 3 and 4, on the otherhand, the solder alloy fails to provide a satisfactory tensile strength.About the elongation, it should be noted that the solder alloycontaining In with a content less than 0.5 wt % as in the case of thecomparative examples 1 and 2 does not provide a satisfactory result,while the solder alloy containing In with a content of 0.5 wt % or moreprovides a satisfactory result.

About the melting temperature, all of the test samples in FIGS. 1-3satisfy the requirement that the melting temperature should be lowerthan 155° C. It should be noted that electronic components are generallydesigned to have a heat-resistant temperature of 183° C. in view of useof conventional solder alloy that contains Pb. By using the solder alloyof the present invention, on the other hand, it is possible to conductthe soldering process at a temperature lower than the temperature usedconventionally, and the problem of thermal damage to the electroniccomponents is minimized. Further, it is possible to reduce the cost ofthe electronic apparatus by simplifying the preparation process ofsoldering as well as by using inexpensive materials for the electroniccomponents.

It should be noted that FIG. 1 further shows a tendency that the meltingtemperature of the solder alloy decreases with increasing In content. Inother words, FIG. 1 indicates that one can control the meltingtemperature of the solder alloy by controlling the In content. Thereby,any necessary change of the soldering temperature depending upon theelectronic component such as a semiconductor device, is easily attendedto.

With regard to the time-to-failure, it will be noted that nosatisfactory result is obtained when the In content is less than 0.5 wt% as in the case of the comparative examples 1 and 2 or when the Incontent exceeds 50 wt % as in the case of the comparative example 3. Itis noted that there is an exception in the case of the comparativeexample 4 in which the time-to-failure falls in the satisfactory rangeof 40-60 minutes even when the In content exceeds 50 wt %. It isbelieved that this exception is caused because of the Bi-freecomposition of the solder alloy that has resulted in an increase of thetime-to-failure.

With regard to the fracture surface of the tested samples, it is notedthat a brittle fracture occurs when the In content of the solder alloyis less than 5.0 wt %. When the In content is equal to or larger than5.0 wt %, a ductile fracture occurs. While the type of fracture of thesolder alloy may not affect the property thereof as a solder, it is morepreferable that the solder alloy shows ductile fracture than brittlefracture in view point of the mechanical strength.

Summarizing the result of FIG. 1, it is concluded that one can obtain asolder alloy of desirable property by incorporating In into a solderalloy of the Sn—Bi eutectic system with a proportion of 0.5 wt % or morebut less than 50.0 wt %. In FIG. 1, the samples 1-1-1-5 provide suchdesirable properties.

In the sample 1-5, it should be noted that, while the tensile strengthis slightly larger than the acceptable lower limit, the solder alloyprovides a much larger elongation over other samples as well ascomparative examples cited in the table of FIG. 1. Thus, by setting thecomposition of the solder alloy such that the solder alloy contains Snwith a proportion of about 34.0 wt %, Bi with a proportion of about 46.0wt % and In with a proportion of about 20.0 wt %, it is possible toobtain a solder alloy having an excellent elongation. Such a solderalloy composition is particularly suitable for soldering components upona flexible substrate where the solder alloy experiences largedeformation.

In the description above, the representation of the composition such as“about 34.0 wt %,” “about 20.0 wt %,” and the like, is used, in view ofpossible error in the composition of the solder alloy that can reach asmuch as ±1 wt % for Sn and Bi and ±0.1 wt % for In.

Next, the result of FIG. 2 will be explained. As already noted, FIG. 2shows the properties of various solder alloys all included in theternary eutectic system of Sn—Bi—In but with various Bi contents and agenerally common In content.

Referring to FIG. 2, it will be noted that a satisfactory tensilestrength is obtained when the Bi content has exceeds 5.0 wt %. About theelongation, no satisfactory result is obtained when the Bi content isequal to or larger than 60.0 wt % as in the case of the comparativeexamples 12-15, while the solder alloy containing Bi with a proportionless than 60.0 wt % provides a satisfactory elongation. About themelting temperature, all of the solder alloy compositions, except forthe example in which the Bi content is 100%, satisfy the requirement. Inother words, it is demonstrated that the melting temperature or liquidustemperature is reduced in the solder alloy that contains Sn, Bi and In.

The result of FIG. 2 further indicates the tendency that the meltingtemperature increases with increasing Bi content. Thus, by adjusting theBi content, it is possible to control the melting temperature of thesolder alloy.

About the time-to-failure, it will be noted that the solder alloyscontaining Bi with less than 5.0 wt %, as in the comparative examples 10and 11, as well as the solder alloys containing Bi with 60.0 wtt ormore, as in the case of the comparative experiments 12-15, provide areduced time-to-failure and hence an unsatisfactory durability. Further,the observation of the fracture surface indicates that the solder alloyshows a ductile fracture when the Bi content is less than 55.0 wt %,while the fracture becomes brittle when the Bi content in the solderalloy is equal to or larger than 55.0 wt %.

Summarizing the result of FIG. 2, it is concluded that a solder alloysuitable for soldering electric and electronic components is obtained byadding Bi to a solder alloy of the Sn—In eutectic system, with aproportion that exceeds 5.0 wt % but smaller than 60.0 wt % as in caseof the examples 2-1 and 2-2 of FIG. 2.

Further, the results of FIGS. 1 and 2 collectively indicate that asolder alloy suitable for soldering electric and electronic componentsis obtained by setting the Bi content to be less than 60.0 wt %, the Incontent less than 50.0 wt %, and by balancing the rest of the solderalloy by Sn. Particularly, one obtains a solder alloy of optimumcomposition by setting the Sn content to about 40.0 wt %, the Bi contentto about 55.0 wt %, and the In content to about 5.0 wt %. As alreadynoted, the phrase “about” is used in view of the possible error in thecomposition when forming the alloy. The error can be as large as ±1 wt %for Sn and Bi and ±0.1 wt % for In.

Next, a description will be made on the experiments conducted by theinventor with reference to FIG. 3, wherein FIG. 3 shows the result ofthe experiments conducted upon a solder alloy based upon the Sn—Bi—Ineutectic system, except that other metal elements, particularly Ag andZn, are added to the foregoing ternary system.

It will be noted that the solder alloy does not satisfy the requirementabout elongation when Ag and Zn are added to the solder alloy of theSn—Bi—In ternary eutectic system with a proportion of 5.0 wt % or morefor each of Ag and Zn. On the other hand, when the proportion of one ofAg and Zn is set to 1.0 wt %, the requirement for elongation issatisfied.

Further, it will be noted in FIG. 3 that all of the samples satisfy therequirement about melting temperature. The result of FIG. 3 indicatesthat one can reduce the melting temperature and hence the liquidustemperature by incorporating Ag and Zn to the ternary solder ally of theSn—Bi—In eutectic system.

The result of FIG. 3 clearly indicates that the solder alloy containingAg and Zn with a proportion of 1.0 wt % or more but below 5.0 wt %, suchas the examples 3-1 and 3-2, satisfies the requirement imposed upon asolder alloy, with every respect of the requirement. Further, it shouldbe noted that the content of Bi and In in FIG. 3 falls in the optimumrange derived from the result of FIGS. 1 and 2. In other words, thecontent of Bi does not exceed 60.0 wt % and the content of In does notexceed 50.0 wt %.

Summarizing the result of FIG. 3, a solder alloy suitable for solderingis obtained from a ternary alloy of the Bi—In—Sn eutectic system bysetting the proportion of Bi and In such that the Bi content does notexceed 60.0 wt %, the In content does not exceed 50.0 wt % and byincorporating Ag or Zn with a proportion equal to or larger than 1.0 wt% but smaller than 5.0 wt %. The rest of the alloy composition isbalanced by Sn. In the embodiment of FIG. 3, it should be noted thatother metal elements such as Ge or Ga may be used in place of Ag and Zn.Further, non-metal elements such as P may also be used for this purpose.

In the ternary alloy composition of the Sn—Bi—In eutectic system shownin FIG. 3, it is also possible to incorporate Sb as an additional metalelement. By adding Sb, the problem of elemental diffusion to a Sn—Pbplating is successfully eliminated. When Pb and Bi are contacted witheach other, there tends to occur a problem of diffusion, which in turnresults in a bulging or coming-off of the solder metal. Thereby, thereliability of the soldering deteriorates significantly. In thelead-free solder alloy of the present invention, such a problem ofdegradation of the solder alloy is successfully eliminated byincorporating Sn as noted above. Thereby, it is preferable to controlthe Sn content in the solder alloy to fall in the range of 1.0-5.0 wt %.The Sn content is optimized in this range in view of the tensilestrength and the elongation of the solder alloy.

Next, a solder alloy of the Sn—Ag—Bi system will be described withreference to FIG. 4 that shows the result of the test conducted upon thetensile strength, elongation, time-to-failure, fracture surfacemorphology and the melting temperature for the solder alloy of variouscompositions. As the tests conducted upon the solder alloys of FIG. 4are identical with the tests described already, further descriptionthereof will be omitted. Similarly to FIGS. 1‥3, FIG. 4 cites theevaluation about the tensile strength, elongation, time-to-fracture andthe melting temperature. When the evaluation is positive, a designationis made by an open circle mark. When the evaluation is negative, on theother hand, a designation is made by a cross mark.

In the test of FIG. 4, a standard is imposed such that a satisfactorysolder alloy should have a tensile strength of 7 kg/mm² or more, anelongation of 7.0 % or more and a melting temperature of 220° C. orless. It will be noted that this standard is different from the standardapplied to the solder alloy containing Sn, Bi and In. The reason ofusing a such different standard is to meet the demand for a solder alloyhaving a particularly large tensile strength. Such a demand on the otherhand does not require a high elongation as in the case of the foregoingsolder alloy of the Sn—Bi—In system.

Referring to FIG. 4, the table shows the properties of the ternarysolder alloy of the Sn—Ag—Bi eutectic system for various contents of Biwhile maintaining the Ag content substantially constant.

From the result of FIG. 4, it will be noted that the solder alloy of theexamples 4-1-4-6 satisfies the foregoing standard. On the other hand,the comparative example 30 that contains Pb has a lower tensile strengthand does not satisfy the foregoing standard, contrary to the ternaryalloy of the Sn—Ag—Bi system. A similar result was obtained also for theexample 31 for the binary alloy of the Sn—Bi eutectic system and for theexample 32 for the binary alloy of the Sn—Ag eutectic system.

About the observed elongation, all of the examples of FIG. 4 satisfy therequired standard. It is known that the elongation and the tensilestrength tend to contradict with each other. Thus, there is a tendencythat an alloy having a large tensile strength shows a small elongation.Under such circumstances, the solder alloy of the present embodimentprovides a particularly high tensile strength while sacrificing theelongation. For example, the samples 4-1-4-6 shows a tensile strengthhigher than that of the comparative examples 30—32 and an elongationsmaller than that of the comparative examples 30—32.

With regard to the melting temperature, all of the examples shown in thetable of FIG. 4 satisfy the required standard. Particularly, theexamples 4-1-4-6 show a melting temperature falling in the range of 139°C.-220° C. It has been practiced, in the conventional lead-containingsolder alloys, to adjust the composition of the alloy such that themelting temperature is held low in view of the endurable temperature of183° C. of the electronic components to be soldered. On the other hand,there also are demands for a solder alloy composition having a highermelting temperature such as 220° C. or more. In order to meet such ademand, there exist a group of solder alloys in which the meltingtemperature is adjusted higher than 220° C. According to the presentembodiment as set forth in the table of FIG. 4, one can provide alead-free solder alloy that is suitable for the purpose whilesimultaneously maintaining a sufficient tensile strength.

With regard to the time-to-failure, there is a tendency that thetime-to-failure increases with increasing elongation. Thus, the examples4-1-4-6 provide a relatively small time-to-failure value as comparedwith the comparative examples 30—32. Even then, the solder alloys of theexamples 4-1-4-6 provides a satisfactory time-to-failure of 230-670seconds while maintaining a large tensile strength.

With regard to the fracture surface, the solder alloy of the presentembodiment shows a feature of brittle fracture. As the solder alloy ofthe present embodiment is intended to provide a high tensile strength,the evidence that the solder alloy shows a brittle fracture does notcause any serious consequence.

Summarizing the result of FIG. 4, it will be noted that the ternarysolder alloy of the Sn—Ag—Bi system provides a superior tensile strengthover the eutectic solder alloys of other compositions describedpreviously such as the examples 30-32, as clearly demonstrated in theexamples 4-1-4-6. Further, it is possible to adjust the meltingtemperature as desired within the temperature range conventionally usedfor soldering. Thus, it is possible to carry out the soldering atvarious temperatures optimized for the components to be soldered whilesimultaneously maintaining a high tensile strength, by selecting thecomposition of the lead-free solder alloy according to the purpose.

In the examples 4-1-4-6 of FIG. 4, it should be noted that the ratio ofthe Sn wt % to the Ag wt % is held constant and only the Bi content ischanged. In other words, the solder alloy composition of the examples4-1-4-6 is represented as 96.5×(100−X)/100 for Sn, 3.5×(100−X)/100 forAg, and X for Bi, all represented in terms of wt %.

From FIG. 4, it is concluded that the following compositions aresuitable for the solder alloy having a large tensile strength: a solderalloy containing Ag with an amount not exceeding 4.0 wt %, Bi with anamount equal to or larger than 1.0 wt %, and Sn with an amount notexceeding 95.0 wt %; a solder alloy containing Ag with an amount between1.0 wt % and 4.0 wt %, Bi with an amount between 1.0 wt % and 40.0 wt %,and Sn with an amount between 55.0 wt % and 95.0 wt %, a solder alloycontaining Ag with an amount of approximately 3.3 wt %, Bi with anamount of approximately 5.0 wt %, and Sn with an amount of approximately91.7 wt %; a solder alloy containing Ag with an amount of approximately3.1 wt %, Bi with an amount of approximately 10.0 wt %, and Sn with anamount of approximately 86.9 wt %; a solder alloy containing Ag with anamount of 3.0 wt %, Bi with an amount of 15.0 wt %, and Sn with anamount of 82.0 wt %; a solder alloy containing Ag with an amount of 2.8wt %, Bi with an amount of 20.0 wt %, and Sn with an amount of 77.2 wt%; a solder alloy containing Ag with an amount of 2.4 wt %, Bi with anamount of 30.0 wt %, and Sn with an amount of 67.6 wt %; a solder alloycontaining Ag with an amount of 2.1 wt %, Bi with an amount of 40.0 wt%, and Sn with an amount of 57.9 wt %, and the like.

FIGS. 5A-5I are diagrams showing the examples of solder particlesforming a solder powder.

Referring to FIG. 5A, the solder alloy of the examples 1-1-1-5 shown inFIG. 1 or the solder alloy of the examples 2-1 or 2-2 of FIG. 2, is usedto form a generally spherical solder particle having a diameter of 20-60μm. FIG. 5B, on the other hand, shows a solder particle formed of thesolder alloy of the example 3-1 or 3-2 of FIG. 3, wherein the solderparticle has a generally spherical form and a diameter of 20-60 μm.Further, FIG. 5C shows a solder particle formed of the solder alloy ofthe examples 4-1-4-6 of FIG. 4, wherein the solder particle has agenerally spherical form and a diameter of 20-60 μm.

FIG. 5D, on the other hand, shows a composite solder particle, in whicha core particle, formed of the solder alloy of any of the examples1-1-1-5 of FIG. 1 or 2-1-2-2 of FIG. 2, is covered by Sn or an alloy ofSn containing Ge with a proportion of 0.1-5.0 wt %, wherein thecomposite solder particle as a whole has a generally spherical form anda diameter of 20-60 μm.

FIG. 5E, on the other hand, shows another composite solder particle, inwhich a core particle, formed of the solder alloy of any of the examples3-1 and 3-2 of FIG. 3, is covered by a similar alloy that contains Sn orGe further with a proportion of 0.1-5.0 wt %, wherein the compositesolder particle as a whole has a generally spherical form and a diameterof 20-60 μm.

Further, FIG. 5F shows another composite solder particle, in which acore particle of the solder alloy of any of the examples 4-1-4-6 of FIG.4, is covered by a similar alloy that contains Sn or Ge further with aproportion of 0.1-5.0 wt %, wherein the composite solder particle as awhole has a generally spherical form and a diameter of 20-60 μm.

FIG. 5G shows another composite solder particle, in which a coreparticle of the solder alloy of any of the examples 1-1-1-5 of FIG. 1 orthe examples 2-1 and 2-2 of FIG. 2 is covered by a similar alloycontaining Sn and Bi with respective proportions exceeding 20.0 wt % andless than 60.0 wt %, wherein the composite solder particle as a wholehas a generally spherical form and a diameter of 20-60 μm.

FIG. 5H shows a still other composite solder particle, in which a coreparticle of the solder alloy of any of the examples 3-1 and 3-2 of FIG.3 is covered by a similar alloy that contains Sn and Bi with respectiveproportions exceeding 20.0 wt % and less than 60.0 wt %, wherein thecomposite solder particle as a whole has a generally spherical form anda diameter of 20-60 μm.

FIG. 5I shows a still other composite solder particle, in which a coreparticle of the solder alloy of any of the examples 4-1-4-6 of FIG. 4 iscovered by a similar alloy containing Sn and Bi with respectiveproportions exceeding 20.0 wt % and less than 60.0 wt %, wherein thecomposite solder particle as a whole has a generally spherical form anda diameter of 20-60 μm.

In any of the embodiments in FIGS. 5A-5I, it is possible to form asolder paste from the solder powder formed of the solder particles.Further, in the embodiments of FIGS. 5D-5I, it is possible to eliminatethe problem of oxidation of the solder alloy by covering the solderalloy by an alloy containing Sn or Ge with a proportion of 0.1-0.5 wt %or by an alloy containing Sn and Bi with the proportion of Sn exceeding20.0 wt % and the proportion of Bi not exceeding 60.0 wt %.

Hereinafter, the solder paste that uses the solder powder of theprevious embodiments will be described.

The inventor of the present invention has prepared various solder pastecompositions, the first series of compositions being a mixture of asolder powder, an amine halide, a polyhydric alcohol and a polymercompound, wherein first series composition contains the solder powderwith a proportion of 80.0-95.0 wt %. Thus, the solder paste compositionof the first series contains, as the remaining component, the aminehalide, the polyhydric alcohol and the polymer compound with aproportion of 20.0-5.0 wt %. The second series composition is a mixtureof a solder powder, an organic acid, polyhydric alcohol and a polymercompound, wherein the second series composition contains the solderpowder with a proportion of 80.0 wt %-95.0 wt %. Thus, the solder pastecomposition of the second series contains, as the remaining component,the organic acid, the polyhydric alcohol and the polymer compound with aproportion of 20.0-5.0 wt %.

As the amine halide for use in the solder paste, one may select one ormore from the group of: acrylic amine hydrochloride, anilinehydrochloride, diethylamine hydrochloride, cyclohexylaminehydrochloride, monomethylamine hydrochloride, dimethylaminehydrochloride, trimethylamine hydrochloride, phenylhydrazinehydrochloride, n-butylamine hydrochloride, 0-methylhydrazinehydrochloride, ethylamine oxalate, cyclohexyl oxalate,2-aminoethylbromide oxalate, and tri-n-butylamine oxalate.

As the organic acid for use in the solder paste, one may select one ormore from the group of: oxalic acid, malonic acid, succinic acid,glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid,sebacic acid, maleic acid, tartaric acid, benzoic acid, acetic acid,hydroxyacetic acid, propionic acid, butyric acid, n-veleric acid,n-caproic acid, enanthic acid, n-capric acid, lauric acid, myristicacid, palmitic acid, stearic acid, and the like.

FIG. 6 shows an example of the solder paste composition that uses asolder alloy described in one of the examples shown in FIGS. 1 through 3as a solder powder, while FIG. 7 shows an example of the solder pastecomposition that uses a solder alloy described in one of the examplesshown in FIG. 4.

It should be noted that the amine halides or organic acids describedabove act as an activating agent. Further, one may use abietic acid,dehydroabietic acid, α-terpineol, and the like, for the base of thepaste. The solder paste may further contain a polymer compound such ascured castor oil as a thixotropic agent. Further, a polyhydric alcoholsuch as 2-methyl 2,4-pentadiol may be added as a solvent.

The solder paste composition described above is naturally free from Pband can be used for a hazard-free reflowing process that does notrequire precautionary measure against toxicity of Pb. Thereby, theefficiency of production of the electronic apparatuses is improved.

FIG. 8 shows an example of application of the lead-free solder alloyupon a conductor pattern on a printed circuit board.

Referring to FIG. 8, a printed circuit board 1 includes a base member 2of a glass-epoxy, wherein the base member 2 carries thereon an electrode3 of Cu for external connection. On the electrode 3, a lead-free solderalloy selected from any of the examples described in FIGS. 1-3 isapplied, such that a film 4 of the solder alloy covers the electrode 3.It was confirmed that such a construction provides an excellent junctionor adherence between the solder alloy and the electrode 3 of Cu, and thefilm of the solder alloy 4 covers the electrode 3 uniformly.

FIG. 9 shows an example in which the solder alloy is applied to coat alead 7 of an electronic component 5 that may be a semiconductor devicehaving a resin package body 6.

Referring to FIG. 9, the lead 7 may be formed of any of Cu, 42-alloy(containing Ni 42 wt %, Co 0.5 wt %, Mn 0.8 wt % and balancing Fe) andCovar, and the lead-free solder alloy of various compositions selectedfrom the examples in FIGS. 1-3 covers the lead 7. It was confirmed thatsuch a construction provides an excellent junction or adherence betweenthe lead 7 and the solder alloy, and a solder alloy film 8 is formed onthe lead 7 with a uniform thickness.

As described above, the lead-free solder alloy successfully coat theconductor patterns on the printed circuit board as well as the terminalsof electronic components, and one can achieve a reliable electrical aswell as mechanical connection between the electronic components and theprinted circuit board.

FIG. 10 shows an example of using the lead-free solder alloy of thepresent invention for the solder bumps that form an external connectionterminal of an electronic apparatus such as a semiconductor device 9.

Referring to FIG. 10, the semiconductor device 9 includes a substrate 13carrying thereon a semiconductor element not illustrated, wherein thesemiconductor element is embedded in a resin package body 14 provided onthe substrate 13. Further, the substrate 13 carries a plurality ofsolder bumps 11 on a lower major surface thereof for externalconnection. Upon placing the semiconductor device 9 on a printed circuitboard 12, the solder bumps 11 engage with corresponding conductorpatterns provided on the printed circuit board 12. Thus, there occurs areflowing of the solder bumps 11 upon passage of the printed circuitboard 12 through a furnace, and a reliable electrical as well asmechanical connection is achieved thereby according to the flip-chipprocess, without using a lead-containing solder alloy.

Next, the soldering process as well as the soldering rig developed forthe lead-free solder alloy of the present invention will be described.

In the foregoing experimental result summarized in FIGS. 1-4, it will benoted that there are examples that show anomalously large elongation asin the case of the examples 1-5 of FIG. 1, 3-2 of FIG. 3 and 4-1 of FIG.4. Further, it was rather frequently observed that the lead-free solderalloys containing Sn and Bi show a rather remarkable increase ofelongation.

The inventor of the present invention at first attributed this effect tothe effect of the impurities contained in the solder alloy. Thus, achemical analysis was conducted upon the solder alloys that showedanomalous elongation by way of the XRF (X-ray fluorescent) analysis andby way of the induction plasma spectroscopy. The result of the chemicalanalysis, however, clearly showed that the solder alloy is essentiallyformed of Sn and Bi and that there is no substantial contamination ofthe solder alloy by a third element.

Based upon the result of the chemical analysis above, the inventor ofthe present invention has set a working hypothesis that the anomalouselongation occurs as a result of the process of preparing the testspecimen, particularly the cooling rate when molding the test piece usedfor the test piece of the specimen.

Thus, the inventor has conducted a series of experiments to mold thetest pieces with various cooling rates, and test the pieces thus formedwere subjected to tests for various mechanical properties such astensile strength, elongation, time-to-fracture, fracture surfaceobservation as well as tests for various metallurgical properties suchas the surface state and metallurgical texture. In the experiments, abinary eutectic alloy having a composition of 42.0 wt % for Sn and 58.0wt % for Bi was used throughout.

When molding the test pieces, three different cooling processes, i.e.,natural cooling process, water cooling process and gradual coolingprocess, were employed. In the natural cooling process, a molten alloywas left in the room temperature environment together with a mold.Thereby, test pieces 5-1-5-3 were obtained according to such a naturalcooling process. In the water cooling process, the mold was cooledcompulsorily by water after molding the test piece from the moltensolder alloy. Thereby, a test piece 5-4 was obtained. In the gradualcooling process, the molten alloy in the mold was gradually cooled byholding the mold in a thermally insulated environment. Thereby, a testpiece 5-5 is obtained.

By employing various cooling processes, the cooling rate of the solderalloy at the time of molding the test piece is changed variously.Particularly, the molding according to the natural cooling process isconducted by setting the mold at various initial temperatures such as200° C. in the case of the example 5-1, 100° C. in the case of theexample 5-2, and 25° C. in the case of the example, 5-3.

In the results of FIG. 11, it should be noted that the mechanicalproperties shown in each of the examples represent the average of threemeasurements conducted upon three test pieces. Thereby, the effect ofscattering of individual measurement is eliminated.

FIG. 12 shows the results of the measurement conducted upon the threetest pieces for the example 5-1. Further, FIG. 13 shows the measurementconducted upon the three test pieces for the example 5-2, FIG. 14 showsthe results of the measurement conducted upon the three test pieces forthe example 5-3, FIG. 15 shows the results of the measurement conductedupon the three test pieces for the example 5-4, and FIG. 16 shows theresults of the measurement conducted upon the three test pieces for theexample 5-5.

FIG. 17 shows the relationship between the load and elongation observedfor the test piece of the example 5-1, wherein the relationship of FIG.17 is for one of the three test pieces that has shown the result closestto the average. Similarly, FIG. 18 shows the relationship between theload and elongation observed for the test piece of the example 5-2,wherein the relationship of FIG. 18 is for one of the three test piecesthat has shown the result closest to the average. FIG. 19 shows therelationship between the load and elongation observed for the test pieceof the example 5-3, wherein the relationship of FIG. 19 is for one ofthe three test pieces that has shown the result closest to the average.FIG. 20 shows the relationship between the load and elongation observedfor the test piece of the example 5-4, wherein the relationship of FIG.20 is for one of the three test pieces that has shown the result closestto the average. Further, FIG. 21 shows the relationship between the loadand elongation observed for the test piece of the example 5-5, whereinthe relationship of FIG. 21 is for one of the three test pieces that hasshown the result closest to the average.

FIG. 22 shows a representative state of fracture of the test piece forthe example 5-1. Similarly, FIG. 23 shows a representative state offracture of the test piece for the example 5-2. Further, FIG. 24 shows arepresentative state of fracture of the test piece for the example 5-3,FIG. 25 shows a representative state of fracture of the test piece forthe example 5-4, and FIG. 26 shows a representative state of fracture ofthe test piece for the example 5-5.

It should be noted that FIG. 11 summarizes the foregoing experimentalresults in FIGS. 12-25.

Hereinafter, the relationship between the cooling condition and themechanical property of the solder alloy will be described with referenceto the experimental results shown in FIG. 11.

First, the effect of cooling process upon the mechanical property willbe examined for the case where the mold temperature is set to 200° C.,based upon the examples 5-1, 5-4 and 5-5.

Referring to FIG. 11, it is clearly indicated that the solder alloyprovides the smallest elongation of 20.44% when the gradual coolingprocess is employed in which the cooling rate is minimum. Withincreasing cooling rate, the elongation increases such that anelongation of 33.67% is obtained as a result of the natural coolingprocess. When a water cooling is employed, an elongation of 89.48% isobtained. The foregoing results indicate that one obtains an increasedelongation with increasing cooling rate.

Next, the effect of the mold temperature on the elongation will beexplained based upon the examples 5-1-5-3 in which the natural coolingis used throughout but with various initial mold temperatures.

Referring to FIG. 11, it will be noted that the highest initial moldtemperature of 200° C., which provides the smallest cooling rate,results in the smallest elongation of 33.67%, while the lower initialmold temperature of 100° C. provides an increased elongation of 137.50%.When the mold temperature is set to 25° C., it is possible to achieve anelongation of 218.33%. This result also supports the conclusion that theelongation increases with increasing cooling rate.

Summarizing the experimental results above, the mechanical properties ofa solder alloy can change variously depending upon the cooling rate,even when the composition of the solder alloy is fixed. With increasingcooling rate, the elongation of the solder alloy increases, and thesolder alloy shows the evidence of ductile fracture.

As will be noted in FIG. 11, the fracture surface of the test piecesthat provide a large elongation, as in the case of the examples 5-2-5-4,do not exhibit a scale-like pattern that is typically observed in thefracture surface of an Sn42—Bi58 alloy cooled slowly. Further, themicroscopic observation of the fracture surface indicates that there isa coarsening of texture in the examples 5-2-5-4. Thus, it is believedthat such a coarsening is responsible for the increase of the elongationof the alloy.

As already noted, the remarkable increase of the elongation occurs notonly in the solder alloy containing Sn and Bi, but also in the alloy ofother compositions. Thus, it is believed that such an increase of theelongation results from the coarsening of texture of the alloy, causedby the large cooling rate.

It should be noted that such a solder alloy composition having a largeelongation is particularly useful in flexible printed circuit boards inwhich the conductor patterns including the solder patterns experiencedeformation.

Hereinafter, a soldering process as well as a soldering rig that carriesout such a soldering process will be described.

FIG. 27 shows the soldering process that uses the lead-free solder alloyof any of the previous embodiments for soldering an electric orelectronic component upon a substrate such as a printed circuit board.Of course, the substrate is not limited to the printed circuit board.

Referring to FIG. 27, a step 10 is conducted at first in which a flux isapplied to the part of the printed circuit board on which the solderingis to be made. The flux is applied for improving the wetting by thesolder alloy, wherein a suitable flux is selected in view of thecomposition of the lead-free solder alloy to be used.

Next, in the step 12, a preheating is conducted upon the printed circuitboard for eliminating inhomogeneity of soldering caused by localizedcooling and associated solidifying of the molten solder alloy.

Further, a step 14 is conducted subsequently, wherein the pre-heatedprinted circuit board is dipped in a bath of molten solder alloy of anyof the foregoing compositions, and the molten alloy covers the exposedconductor pattern as well as the lead or electrode of the electric orelectronic components. Thereby, the soldering is achieved. The steps10-14 are substantially the same as the conventional soldering processthat uses a lead-containing solder alloy.

Next, in the following step 16, the printed circuit board is pulled upfrom the solder bath and cooled by suitable external cooling means, suchthat the solder alloy experiences a rapid cooling or quenching. As aresult of such a rapid cooling, the solder alloy shows an improvedelongation as explained already. As the external cooling means, one mayemploy a jet of cooling medium such as a coolant gas or volatile organicsolvent. Such a jet of cooling medium can be applied selectively to thepart where the soldering has just been made.

FIG. 28 shows the construction of a soldering rig 20 according to anembodiment of the present invention for conducting the soldering processof FIG. 27. It should be noted that the soldering rig 20 is designedprimarily to carry out a soldering of a sheet-like or plate-like objectsuch as a printed circuit board. However, the soldering rig is by nomeans limited to such a soldering of printed circuit boards but isapplicable to various soldering processes.

Referring to FIG. 27, it will be noted that the soldering rig 20includes a flux coating unit 21, a pre-heating unit 22, a soldering unit23, a transport conveyer 24 and a cooling unit 25 that characterizes therig 20 of the present invention, each of which will be explained below.

The transport conveyer 24 carries a printed circuit board 26 placedthereon and transports the same in a direction indicated by an arrow.Further, the flux coating unit 21, the pre-heating unit 22, thesoldering unit 23 and the cooling unit 25 are disposed consecutivelyalong the transport conveyer 24 in the transport direction of theconveyer 24.

Thus, the flux coating unit 21 applies a flux upon the printed circuitboard 26 and the pre-heating unit 22 preheats the printed circuit board26 thus applied with the flux. Further, the soldering unit 23 carriesout the soldering by means of the lead-free solder alloy describedpreviously.

After the soldering, the printed circuit board 26 is forwarded to thecooling unit 25 by the transport conveyer 24. Thereby, the cooling unit25 rapidly cools the high temperature solder alloy applied by thesoldering unit 23. As a result of such a rapid cooling, it is possibleto increase the elongation of the solidified solder alloy as explainedalready.

FIGS. 29-31 show the construction of the cooling unit 25.

Referring to FIG. 29 showing an example 25A of the cooling unit 25 thatuses liquid nitrogen as a cooling medium, the cooling unit 25A includesa tank 27 for containing liquid nitrogen wherein the liquid nitrogen inthe tank 27 is supplied to an evaporator 28 that evaporates the liquidnitrogen and forms a low temperature nitrogen gas. The low temperaturenitrogen gas thus formed, in turn, is supplied along a pipe 29 to whichone or more gas nozzles 30 are connected. Thereby, the low temperaturenitrogen is injected upon the location of soldering on the printedcircuit board for cooling the high temperature solder alloy.

FIG. 30 shows another example 25B of the cooling unit 25, wherein thecooling unit 25B uses a volatile freon gas as a cooling medium.

Referring to FIG. 30, the cooling unit 25B includes a tank 31 of freonto which a supply pipe 33 of freon is connected. Further, one or morenozzles 32 are connected to the pipe 33 for injecting the freon upon theprinted circuit board 26 on the conveyer 24 at the location where thesoldering has just been made. Thereby, the high temperature solder alloyexperiences a rapid cooling upon the evaporation of freon.

FIG. 31 shows another example 25C of the cooling unit 25 that isdesigned to cool a cylindrical or tubular object after soldering. As thecooling unit 25C of FIG. 31 employs the construction of FIG. 29, thoseparts corresponding to the parts shown in FIGS. 29 are designated by thesame reference numerals and the description thereof will be omitted.

Referring to FIG. 31, the cooling unit 25C includes an annular nozzleelement 35 in which a plurality of nozzles 36 are provided. The nozzleelement 35 is disposed in a tube 37 adapted for passing a cylindricalobject 34 that has experienced soldering, for example by means of asoldering iron 38 that uses a lead-free solder alloy 39 of the Sn—Bieutectic system. Thereby, the object 34 is cooled upon passage throughan inner space of the annular nozzle element 35. As the nozzles 36 aredisposed with a generally uniform interval on the nozzle element 35, theseam of the cylindrical object 34 where the soldering has been made,experiences a uniform cooling by the low temperature nitrogen gasinjected from the nozzles 36.

Further, the present invention is by no means limited to the embodimentsdescribed heretofore, but various variations and modifications may bemade without departing from the scope of the invention.

What is claimed is:
 1. A lead-free solder alloy composition containingSn, Ag and Bi, with respective concentrations set such that saidlead-free solder alloy has a melting temperature lower than apredetermined heat-resistant temperature of a work to be soldered,wherein said lead-free solder alloy composition contains Ag with anamount of approximately 3.1 wt %, Bi with an amount of approximately10.0 wt %, and Sn with an amount of approximately 86.9 wt %.
 2. Alead-free solder alloy composition containing Sn, Ag and Bi, withrespective concentrations set such that said lead-free solder alloy hasa melting temperature lower than a predetermined heat-resistanttemperature of a work to be soldered, wherein said lead-free solderalloy composition contains Ag with an amount of 3.0 wt %, Bi with anamount of 15.0 wt %, and Sn with an amount of 82.0 wt %.
 3. A lead-freesolder alloy composition containing Sn, Ag and Bi, with respectiveconcentrations set such that said lead-free solder alloy has a meltingtemperature lower than a predetermined heat-resistant temperature of awork to be soldered, wherein said lead-free solder alloy compositioncontains Ag with an amount of 2.8 wt %, Bi with an amount of 20.0 wt %,and Sn with an amount of 77.2 wt %.